If you are studying for a future LSAT, I would HIGHLY RECOMMEND that you go take this free LSAT regardless of your skill level. As Vera pointed out, it still involves simulated test conditions. You can't reproduce these on your own unless you have several friends who want to pose as annoying test takers and another really good friend with LSAT experience and the patience to adminster 5 sections for you. That said, they also give you a nice score report that breaks down your performance on different types of questions so you can see what you need to work on. I know the first Kaplan diagnostic I took I did terrible on--a direct result of distractions by others, more strict proctoring, a little bit of nervousness, and just getting use to a different desk than I usually practice on.
Not to be a jerk, but I think it will also be a good wake-up call to those of you who "proctor" your own exams and allow yourself to get away with stuff that test administrators will not (i.e...use an extra 5 seconds to finish a section, bubble in your guesses after time is called, take more than a few seconds for breaks between sections, score your own tests and give yourself a question or two because you ALMOST chose the right one or forgot to guess a letter on, etc.). That 175 you're getting can quickly become a 165 if you don't do everything exactly right. The Kaplan simulation will let you know what will and won't be allowed so that when you practice on your own you can adjust your own proctoring rules accordingly.